1. Field of the Invention
The present invention relates to an optical disk and a method for recording and reproduction of various information on a recording medium such as an optical disk by directing laser beams onto the recording medium, and more particularly it relates to an optical disk and a method of tracking on the disk wherein the recording and reproduction with a high density and a high transfer rate can be realized by narrowing the track pitch of the optical disk.
2. Description of Prior Art
Various optical disk devices have recently been proposed for recording and reproduction of many types of information on the disk by using laser beams. The recording density and transfer rate however have not been greatly improved. The reason for this is that a space of about a laser wavelength is commonly provided between tracks in order to suppress crosstalk between tracks. In view of the above, a recording and reproduction method for doubling the recording density and transfer rate by removing the space between tracks has been proposed (Japanese patent application No. 57-147133). In the method, the cross section of tracks along the radial direction of the disk is formed to have V-shape grooves, and two focussed laser spots are independently driven and projected onto the two adjoining slants on both sides of a V-shape groove in order to record and reproduce signals.
FIG. 1 shows a perspective view of a cross section cut along the radial direction of an optical disk having V-shape grooves. Onto the central portions of two adjoining slants such as A and B, or C and D in FIG. 1, laser spots 1 and 2 are projected as shown in FIG. 2. By driving the two laser beams independently from each other, it is possible to record separate signals on the two slants.
Next, a method of reproducing signals recorded as above will be described in brief. FIG. 3 is a sectional diagram of FIG. 2 as cut along the radial direction of the disk. Similarly to FIG. 2, laser spots 1 and 2 (which are depicted as a laser beam intensity distribution in FIG. 3) are projected on the slants C and D, the distance P between the centers of the spots being arranged to be maintained constant. As disclosed by the above Japanese patent application, by properly setting the shape of V-grooves and detecting mainly .+-.1st-order diffraction beams among the beams reflected from the disk, it is possible to reproduce respective signals from the slants. The distribution of reflected light beams in the above case is shown in FIG. 5(a). Numeral 3 represents the surface of a lens (exit or entrance pupil) which focusses laser beams onto the disk. In the case that a reproducing laser beam is projected only at the central portion of the lens surface (entrance pupil), the distribution of 0-order diffraction beam E.sub.O and .+-.1st-order diffraction beams E.sub.+1, E.sub.-1 among the light beams reflected from the disk on the lens surface (exit pupil) is as shown in FIG. 5(a). Therefore, by disposing photosensors A and B as shown in FIG. 5(b), for example, and detecting reflected lights at the respective peripheral portions of the reflected light beam 4, each signal recorded on each slant can be independently reproduced.
Next, a tracking method in the above described recording and reproducing method will be explained (as disclosed by Japanese patent application No. 58-17529; U.S. patent application No. 525,411 was filed on the basis of the two Japanese application Nos. 57-147133 and 58-175259). The sectional view of an optical disk for use with the tracking method is shown in FIG. 3. As shown in FIG. 3, the top or uppermost portions of V-shape grooves are formed such that the depths of the grooves are made to slightly vary at two different frequencies f1 and f2 alternately in order to constitute pilot signals. These frequencies are set for example lower than the frequencies of signals recorded on the slants. The intensities of lights reflected from the disk are respectively modulated by the frequencies f1 and f2. In particular, if a tracking laser spot deviates toward the slant E of FIG. 3, the reflection beam includes an increasing component modulated by the frequency f2 and a decreasing component modulated by the frequency f1. Conversely, if a tracking laser spot deviates toward the slant B in FIG. 3, the component of f1 increases and the component of f2 decreases. Accordingly, by controlling the reflection beams so as to include equal amounts of components varying at the two frequencies f1 and f2, the two laser spots can correctly track on the slants C and D. In the laser spots' tracking on the slants A and B, it is necessary to reverse the polarity of the control because, as shown in FIG. 3, the right and left positional relation of the slants B and A for the respective frequencies f1 and f2 is made contrary to that of the slants C and D.
On this account, in the case of a disk having spiral tracks, in order to change the tracking control polarity for every one track, signals must be provided for indicating the positions where the control polarity is changed. One example of a disk having spiral tracks is shown in FIG. 4. The solid line indicates the bottom of the V-shaped groove along which the middle point of two laser spots traces. As seen from the figure, the tracks change their depths at the frequencies f1 and f2 alternately at every revolution of the disk. An area W is provided to form the V-shaped grooves varying their depths at a third frequency f3 so that upon detection of the frequency f3 from the reflected beams, the switching of tracking control polarity can take place. This method however has a problem of unstable tracking since the tracking control in association with the frequencies f1 and f2 can not be performed in the area W. Further, with the method, detection means must be provided for detecting from the reflected beams the three kinds of frequency components f1, f2, and f3. As a result, a circuit arrangement of large scale is required and it is not suitable from the view point of cost. In addition, the formation of such tracks with groove depths modulated by three frequencies becomes one of the factors complicating the manufacturing process of the disks.
The method incorporating therein the switching of tracking control has such problems. Therefore, a development on tracking methods without the control switching has long been desired.
FIG. 6 shows a cross sectional view including slants C and D. In FIG. 6, P represents a track pitch, e.g., P=800 nm. .theta. represents an angle between the slants C and D, e.g., .theta.=163.degree.. In this case, if the depths of respective slants are varied at frequencies f1 and f2 and with an amplitude of .delta.=30 nm, then the juncture line between the slants C and D shifts with an amplitude .gamma.=300 nm in the radial direction of the disk and it wobbles with revolution of the disk. Therefore, the practical width of available recording area becomes R=700 nm which is narrower by 12.5% than the track pitch P. As mentioned above, the method changing the depth of every track has a disadvantage that the practically available recording area becomes smaller.